The present invention belongs to the field of medical diagnosis using an ultrasonic wave and, more particularly, it relates to an ultrasonic diagnosing apparatus in which a tomogram of a subject under examination is obtained due to a pulse-echo method, and blood flow information is derived due to a continuous wave Doppler method.
A Doppler ultrasonic method which is used to obtain blood flow informatioin is mainly classified into a modulated wave Doppler method using the modulated (amplitude modulated) ultrasonic wave and a continuous wave Doppler method using a continuous wave of the ultrasonic wave.
A typical modulated wave Doppler method is a pulsed Doppler method. In the pulsed Doppler method, a pulsed ultrasonic wave forming a burst wave having a pulse-like envelope is sent into a subject under examination, and the blood flow information is measured from an echo component (returned component) due to the reflection of the ultrasonic wave by (a hemocyte of) a blood flow. This blood flow information is measured by measuring the influence due to the Doppler effect in the echo component. Advantages of this pulsed Doppler method include the possibility of commonly using a probe and a transmitter/receiver in a tomograph to obtain an ultrasonic tomogram (B mode image) by the pulse-echo method (since a pulsed wave is used in both pulsed Doppler method and pulse-echo method), and the blood flow information at an arbitrary position on the tomogram is derived, etc.
However, in the modulated wave Doppler method, there is a limitation in principle regarding the detection range of the blood flow velocity, and it is difficult to identify a high blood flow velocity. Namely, first, the frequency deviation of only up to half of the repetition frequency of the ultrasonic pulse can be accurately detected according to this modulated wave Doppler method. Therefore, it is impossible to measure the blood flow velocity exceeding the velocity corresponding to the above frequency deviation. Second, according to the modulated wave Doppler method, when measuring the blood flow velocity at a portion in a particular depth of a subject, the cyclic period of the ultrasonic pulse cannot be shorter than the time the ultrasonic wave travels to that depth and back to the transducer. Because of these reasons, it is difficult to identify a high blood flow velocity.
On the other hand, in the continuous wave Doppler method, a non-modulated continuous ultrasonic wave is sent into a subject and blood flow information is obtained on the basis of the echo signal received by another transducer other than a transducer used for transmission of a wave; therefore, there is not such a limitation regarding the detection range as mentioned above.
However, the continuous wave Doppler method needs independent transducers for transmitting and receiving a wave, respectively, so that the arrangement becomes complicated. Further, in the continuous wave Doppler method, since a continuous wave is used, the resolution is not provided with respect to the distance from the transducers for transmission and reception, that is, with regard to the direction along the ultrasonic beam, so that it is difficult to identify the measuring position.
As an ultrasonic probe used in the continuous wave Doppler method, for example, there is known a type such as shown in U.S. Pat. No. 4,413,629. FIG. 1 schematically shows an ultrasonic probe of this type. An ultrasonic probe 1 has a transmitting transducer 2 and a receiving transducer 3 which are respectively fixed at the distal end.
However, since the transducers 2 and 3 of the probe 1 are fixed, the measuring areas which are determined due to the directivity of the transducer 2 for transmission and due to the directivity of the transducer 3 for reception are fixed. Namely, the measuring area is the intersecting area of the transmission beam from the transducer 2 for transmission and the receiving beam of the transducer 3 for reception. (The ultrasonic wave which is received can be regarded as a ultrasonic beam since the transducer has wave-reception directivity. This particular ultrasonic beam is called the receiving beam.) When the transducers 2 and 3 are fixed, both beam positions to the probe 1 are fixed, so that the intersecting area of both beams is fixed. Therefore, when using this probe 1, a plurality of probes which were respectively set to various distances between the probe and the measuring area are preliminarily prepared, and the probe in accordance with the depth of the measuring position must be selected from among these plurality of probes and has to be used. In addition, the probe for the measurement of the blood flow due to this Doppler method is constituted separately from (quite irrespectively and independently of) a transducer array for a tomogram (not shown) to obtain tomograms; therefore, there are drawbacks such that it is difficult to comprehend the corresponding relation with the tomograms, and it is also difficult to clearly discriminate from which portion in a subject under examination the measured blood flow information was obtained, and the like.
On the other hand, an ultrasonic probe 4 as shown in FIG. 2 is considered. A transducer array 5 for picking up tomograms, a single transducer 6 for transmitting a continuous wave for measuring the blood flow due to the Doppler method, and a single transducer 7 for receiving this continuous wave are respectively fixed to the point of the probe 4. In the transducer array 5, a plurality of transducer elements are arranged in a line. Such a probe can relatively easily grasp the corresponding relation with the tomograms. However, since there is a limitation regarding the overall size of the probe 4, mainly in consideration of the easiness of use, there is a tendency such that the areas of the transmission surface of the transducer 6 and of the reception surface of the transducer 7 become insufficient. In such a case, the sensitivities of the transducers 6 and 7 deteriorate, so that accurate blood flow information cannot be obtained. On the other hand, similar to the case of the probe 1 shown in FIG. 1, since both transducers 6 and 7 are fixed, the intersecting area of the transmitting beam and receiving beam is fixed, so that it is impossible to obtain stable sensitivity from a shallow portion to a deep portion in the subject.